(a) Field of the Invention
This invention relates to a novel process for preparation of benzothiazolylsulfenamides. More Particularly, it relates to a process for preparation of benzothiazolylsulfenamides which comprises cross-coupling, via direct electrolytic oxidation in the presence of a solvent and a suitable supporting electrolyte, 2-mercaptobenzothiazole or dibenzothiazyl disulfide or an alkali metal salt of 2-mercaptobenzothiazole with a primary or secondary amine containing an alkyl group or groups having 2 to 8 carbon atoms or a heterocyclic ring.
(B) Description of the Prior Art
The benzothiazolyslsulfenamides to be used as vulcanization accelerators for rubber have been conventionally prepared by charging into a reactor, 2-mercaptobenzothiazole or dibenzothiazolyl disulfide which is an oxidation product of the benzothiazole, a primary amine such as tert-butyl amine, cyclohexyl amine or a secondary amine such as diethylamine, diisopropylamine, morpholine, and a suitable solvent, and adding thereto an oxidizing reagent such as sodium hypochlorite solution dropwise thereby to effect oxidation. The reaction process may differ somewhat according to the kinds of the amines employed in the reaction.
For example, when cyclohexylamine is used as the amine, a reactor equipped with cooling means is charged with 2-mercaptobenzothiazole, cyclohexyl amine, and water, and the mixture is stirred to form a cyclohexylamine salt of 2-mercaptobenzothiazole. Sodium hypochlorite solution is then added to the reaction mixture dropwise to effect chemical oxidation while the reactor is cooled with water since the reaction is exothermic. The resulting ash-colored or yellowish ash-colored precipitation is subjected to filtration, rinsing with water, dewatering, drying and grinding, to obtain N-cyclohexyl-2-benzothiazolylsulfenamide.
When heterocyclic morpholine is used as the amine, morpholine and a suitable solvent are charged into a reactor, and thereafter sodium hypochlorite solution is added thereto to form morpholine chloride. Then, to the reaction mixture are added dibenzothiazyl disulfide and morpholine, and then sodium perchlorate (an oxidizing reagent) is further added dropwise to effect chemical oxidation. This is followed by removal of morpholine in excess as well as the solvent. The resulting pale brownish yellow precipitation is subjected to filtration, rinsing with water, dewatering, drying, and grinding to obtain N-oxydiethylene-2-benzothiazolylsulfenamide.
Such conventional processes are very complicated since the previous-stage reaction is required before the oxidation reaction. Moreover, the yield is at most about 90% to 93% since a side reaction cannot be avoided in the course of the oxidation reaction. In order to improve the quality of the product and to increase the yield to some extent, it is necessary to simplify the complicated reaction steps accompanied by various production-control problems. Also, it is further necessary to take a measure against waste water pollution caused by side-reaction products as well as small amounts of solvents and the starting amine which could not be recovered or removed and to lower the heavy expenses for a waste water-disposal plant investment as well as the operating cost.
As a result of research, it has now been found that the difficulties of the prior art process can be readily overcome by employing electrolytic oxidation reaction. The present invention is based on this discovery.